include file("./provide-data.arr")
  cases(BT) mt:
    | mt => print("a")
    | node(_, _, _) => print("b")
  end

check:
  t :: BT = mt
  mt satisfies is-object
  node satisfies is-function
  is-mt satisfies is-function
  is-node satisfies is-function
  is-BT satisfies is-function

end
